Surface exclusion mechanism

In which the ratio m/n is close to 3. The silane was produced by free radical copolymerization of vinyltriethoxysilane with N-vinylpyrrolidone. Its number-average molecular weight evaluated by vapour-phase osmometry was 3500. Porous silica microballs with a mean pore diameter of 225 A, a specific surface area (Ssp) of 130 m2/g and a pore volume of 0.8 cm3/g were modified by the silane dissolved in dry toluene. After washings and drying, 0.55% by weight of nitrogen and 4.65% of carbon remained on the microballs. Chromatographic tests carried out with a series of proteins have proved the size-exclusion mechanism of their separation. 

The restricted access principle is based on the concept of diffusion-based exclusion of matrix components and allows peptides, which are able to access the internal surface of the particle, to interact with a functionalized surface (Figure 9.2). The diffusion barrier can be accomplished in two ways (i) the porous adsorbent particles have a topochemically different surface functionalization between the outer particle surface and the internal surface. The diffusion barrier is then determined by an entropy controlled size exclusion mechanism of the particle depending on the pore size of adsorbent (Pinkerton, 1991) and (ii) the diffusion barrier is accomplished by a dense hydrophilic polymer layer with a given network size over the essentially functionalized surface. In other words, the diffusion barrier is moved as a layer to the interfacial... 

A RAM column functions through a size exclusion mechanism. Large biomolecules such as proteins are restricted from the adsorptive surfaces inside silica particles. Small analyte molecules are able to penetrate into the inner surfaces of the particles. As a result, protein molecules pass through the column rapidly and analytes of interest are retained on the adsorptive sites. Depending on the application, the analyte molecules are directed to MS for detection or transferred onto an analytical column for separation prior to MS detection. Detailed applications are discussed in a recent review.8... 

Sander et al. [63] investigated the effect of microparticulate silica pore size on the properties of solution-polymerized Cig stationary phases and observed both an increase in bonding density and shape recognition for wider pore (>120 A) silica. A size-exclusion mechanism was proposed, in which the reaction of the silane polymer on the surface is enhanced for wide pores and reduced for narrow pores. Polymeric Ci8 phases prepared on substrates with narrow pores exhibited monomeric-like chromatographic properties. This effect may be the result of an increase in competitive surface linkage with the less sterically hindered monomers that coexist with the bulkier oligomers that have polymerized in the reaction solution (Figure 5.13). 

Cryo- and Lyoprotectants and Bulking Agents Various mechanisms are proposed to explain why excipients serve as cryo- or lyoprotectants. The most widely accepted mechanism to explain the action of cryoprotection is the preferential exclusion mechanism [177]. Excipients that will stabilize proteins against the effects of freezing do so by not associating with the surface of the protein. Such excipients actually increase the surface tension of water and induce preferential hydration of the protein. Examples of solutes that serve as cryoprotectants by this mechanism include amino acids, polyols, sugars, and polyethylene glycol. 

Several authors reported measurements of the preferential binding parameter in the system water (l)/protein (2)/PEG (3) [10—14]. It was found that for various proteins, various PEGs molecular weights, and various PEG concentrations, the protein is preferentially hydrated and the PEG is excluded from the vicinity of the protein molecule. The prevalent viewpoint which explains such a behavior is based on the steric exclusion mechanism suggested by Kauzmann and cited in Ref. [15]. According to this mechanism [12,14], the deficit of PEG and the excess of water (in comparison with the bulk concentrations) are located in the shell (volume of exclusion) between the protein surface and a sphere of radius R (see Fig. 1) [12,14]. However, Lee and Lee [10,11] suggested that the preferential exclusion of the PEG from the protein surface also involves the protein hydrophobicity and charge. 

Schubert S and Lauchli A (1990) Sodium exclusion mechanisms at the root surface of two maize cultivars. Plant Soil 123 205 - 209. 

Models considering diffusion in the bulk as the only rate controlling process are called pure diffusion controlled. When the diffusion is assumed to be fast in comparison to the transfer of molecules between the subsurface and the interface the model is called kinetic-controlled or barrier-controlled. Both steps are taken into account in so-called mixed diffusion kinetic controlled models. Van den Tempel proposed processes within the adsorption layer to be considered instead of hypothetical adsorption barriers [18, 19, 20]. We believe that such models, which account for actual physical processes within adsorption layers, such as reorientation of molecules, their dimerisation and formation of clusters, although explanations for all known cases of anomalous adsorption kinetics do not exist yet, have to be preferred over any formal model. However, reliable experimental evidence for a slower surface tension decrease caused by aggregation within the adsorption layer does not allow the conclusion that this is an exclusive mechanism. 

The basis of RAMs is the simultaneous size exclusion of macromolecules and extraction/enrichment of low-molecular-weight compounds into the interior phase via partition. The outer surface of the particles, which is in contact with biological matrix components such as proteins and nucleic acids, possesses a special chemistry to prevent adsorption of these molecules. Macromolecules can be excluded by a physical barrier, the pore diameter, or by a chemical diffusion barrier created by a protein (or polymer) network at the outer surface of the particle. RAMs can be classified according to the protein exclusion mechanism used into the following two groups RAM with a... 

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